1. Field of the Invention
The present invention relates to ECL circuits in general and to a method and apparatus to compensate for the negative temperature coefficient of the V.sub.BE of an unbiased ECL output transistor in particular.
2. Description of Prior Art
Typically, the potential on the output, i.e. emitter, of an unbiased ECL output transistor is designed to switch between a high output potential VOH of -0.8 volts and a low output potential VOL of -1.6 volts. However, in actual practice, both the VOH and the VOL may be higher or lower depending on the operating temperature. For example, as the temperature of the output transistor increases, the V.sub.BE of the transistor decreases causing both VOH and VOL to increase for a given signal potential applied to the base thereof.
In most circuits, the practical maximum and minimum allowable limits of both VOH and VOL, which define VOH and VOL guard bands, respectively, increase linearly with temperature.
When designing an ECL circuit using an unbiased output transistor, the designer attempts to design the output transistor such that its VOH and VOL will not exceed the minimum and maximum VOH and VOL guard band limits. To do this, the designer typically optimizes the design of the output transistor to provide the desired VOH and VOL at the expected normal operational temperature. However, as a practical matter, it is found that the V.sub.BE of the output transistor comprises a negative temperature coefficient having a slope which is typically greater than the slope of the guard bands such that at elevated temperatures, e.g. 150.degree. C., VOH and VOL closely approach or exceed the maximum VOH and VOL guard band limits. Conversely, at lower than normal temperatures, VOH and VOL closely approach or exceed the minimum VOH and VOL guard band limits. Such conditions place undesirable constraints on the operational characteristics of the ECL circuit.